A radiosonde is a measuring device, which is used in measurements of climatic conditions. The radiosonde may be carried upwards from ground surface through layers of air with a special gas balloon, whereby the atmosphere to be measured and also the environmental conditions of the measurement arrangement change dynamically. Meteorological measurements are performed with measuring devices in the radiosonde, in which case variables to be measured are generally temperature, humidity, pressure and wind (and/or location). Determining the location of the radiosonde may be based on, for example, the GPS (Global Positioning System) or Loran network methods, and furthermore, the velocity and direction of the wind may be deducted on the basis of changes in location information. The measuring data of measuring devices are communicated with a radio transmitter in the radiosonde via a radio receiver on the ground to ground equipment for further processing. The analysing of measurement results is performed, for example, by means of a suitable computer software/algorithm on the ground surface or in the radiosonde.
The environmental conditions of radiosonde measurements are demanding: the measurements are complicated by, inter alia, large scale of measured variable, rain, humidity, freezing, condensation and oversaturation. Measuring errors are caused also by slowness of radiosonde measurement sensors (time delay) and radiative heat exchange occurring in the atmosphere, such as solar radiation and, at night, infrared radiation (IR).
In order to improve the accuracy of temperature measurements performed with the radiosonde, methods have been developed to eliminate errors from the temperature measurement results caused by, inter alia, time delay and radiative heat exchange.
Humidity measurement is more demanding already as such and because humidity measuring is also dependent on temperature in addition to the above mentioned error factors. Humidity measurement is calibrated to the humidity and temperature range used, whereby the measurement includes a correction of temperature dependency (the sensor and measured air are in the same temperature). However, known corrections of humidity measuring do not include a correction of humidity measurement results of the radiosonde in relation to errors especially resulting from radiative heat exchange directly or indirectly.
Recent development in forecast and research of changes in climatic conditions and of traditional weather forecast have set even more strict accuracy requirements for humidity measurement results, especially in the upper troposphere and stratosphere in which low temperatures combined with low water vapour concentration (stratosphere) or high water vapour content (upper troposphere) make the measuring environment extremely challenging. Humidity also varies quickly in relation to time and location. The accuracy of humidity measurements is an important quality factor in weather forecasting. Also in understanding the greenhouse effect and forecasting the changes in atmosphere, humidity measurement is an essential factor.
Typically, the humidity measurement results produced by different radiosondes have not been, as such, accurate enough for, for example, exact research and forecast related to climatic conditions. Thus, it is necessary to improve the accuracy of humidity measurement results.